The present invention relates to a method for determining the quantity of alloy phases (which correspond to a xcex6 phase, a xcex41 phase, and a xcex93 phase in alloyed hot-dip galvanized steel sheets) in plating layers of plated metal materials and a method for evaluating the sliding property thereof.
Among plating layers of plated metal materials, a plating layer having a single phase of a metal and another plating layer having different kinds of alloy phases are known.
Particularly, in plated products having different kinds of alloy phases, it is known that various characteristics of the products significantly depend on the compositions and the quantity of the alloy phases.
Thus, the control of the alloy phases is essential to improve the plating characteristics.
A plating layer of an alloyed hot-dip galvanized steel sheet, of which the production is large among surface-treated steel sheets, has Znxe2x80x94Fe alloy phases and is typical of plating layers having different kinds of alloy phases.
In the alloyed hot-dip galvanized steel sheets described above, the alloy phases having a significant influence on the plating characteristics correspond to Znxe2x80x94Fe alloy phases (a xcex6 phase, a xcex41 phase, and a xcex93 phase) Particularly, the xcex6 phase has a significant influence on the sliding property of alloyed hot-dip galvanized steel sheets suitable for rustproof steel sheets for automotive bodies.
In order to analyze the structure of the alloy phases of plated steel sheets, among physical techniques, the observation of a cross section of a steel sheet with an optical microscope or a scanning electron microscope is performed in general (Akihiko NISHIMURA, Jun-ichi INAGAKI, and Kazuhide NAKAOKA, Tetsu-to-Hagane, 8,101 (1986)).
According to such observation, although the degree of the growth of each alloy phase can be obtained qualitatively and the average thickness of each alloy phase can also be obtained quantitatively, there is a problem in that the preparation of samples and the observation are troublesome.
Since plated products have recently been required to have higher performance, there is a problem in that a small quantity of alloy phases adversely affects the plating characteristics.
That is, in alloyed hot-dip galvanized steel sheets, although the formation of a xcex6 phase and a xcex93 phase should be suppressed, it is difficult to identify such a small quantity of these alloy phases.
On the other hand, the relationship between the diffraction intensity of each alloy phase and the plating characteristics has been studied using X-ray diffraction, and such a technique is intended to apply analysis in a production line.
For alloyed hot-dip galvanized steel sheets, the relationship between the intensity of X-ray diffraction of each alloy phase and the sliding property or the anti-powdering property during the processing of plated steel sheets has been reported (Masato YAMADA, Aki MASUKO, Hisao HAYASHI, and Naoki MATSUURA, Current Advances in Materials and Processes, 3,591 (1990)). The application of an X-ray diffraction method to an analysis in a production line has also been reported (Junji KAWABE, Tadao FUJINAGA, Hajime KIMURA, Kazuya OSHIBA, Tadahiro ABE, and Toshio TAKAHASHI, KAWASAKI STEEL GIHO, 18,129 (1986)).
However, these techniques cannot directly give the absolute quantity of each alloy phase. In order to determine the quantity of each alloy phase in these techniques, it is necessary to make calibration curves using standard samples, of which the content of each alloy phase is known, to obtain the content on the basis of the ratio of the intensity of a measuring sample to the intensity of a standard sample.
That is, in order to determine a small quantity of a xcex6 phase or a xcex93 phase in, for example, alloyed hot-dip galvanized steel sheets, it is necessary to obtain standard samples of which the content of a xcex6 phase or a xcex93 phase is known.
On the other hand, among chemical methods, constant current anodic electrolysis (electrolytic stripping) is used. In this method, a time of a plateau of a potential corresponding to each alloy phase is obtained using a time-potential curve to determine the thickness of each alloy phase according to the quantity of electricity (S. C. Britton, J. Inst. Metals, 58,211 (1936)).
In the above method, since inflection points of the potential are not clear in alloyed hot-dip galvanized steel sheets having a small quantity of a xcex6 phase and a xcex93 phase, it is difficult to determine a small quantity of phases such as a xcex6 phase and a xcex93 phase.
Furthermore, in this method, it is difficult to dissolve each alloy phase in a plating layer uniformly.
It is reported that the direct conversion of a time of a potential plateau into the thickness of a plating layer is not correct due to the residual xcex93 phase having a high content of iron when applying the above method to the analysis of alloyed hot-dip galvanized steel sheets, (Susumu KUROSAWA, the Journal of the Surface Finishing Society of Japan, 45,234 (1994)).
Since the shape of the time-current curve changes depending on the surface state of a sample, it is further difficult to determine a small quantity of an alloy phase, for example, a xcex6 phase, situated near the surface of a plating layer in a alloyed hot-dip galvanized steel sheet.
In order to solve the above problems of conventional techniques, it is an object of the present invention to provide a method for directly determining the quantity of alloy phases (which corresponds to a xcex6 phase, a xcex41 phase, and a xcex93 phase in alloyed hot-dip galvanized steel sheets) in a plating layer with preciseness and a method for evaluating the sliding property of an alloyed hot-dip galvanized steel sheet.
A first aspect of the present invention provides a method for determining a quantity of each of alloy phases in a plating layer, wherein the method includes subjecting each alloy phase in a plating layer to constant potential electrolysis in each of a plurality of ranges of potentials obtained on the basis of the immersion potential of each alloy phase and the immersion potential of a basis metal, by using a plated metal material having different kinds of alloy phases in the plating layer as the anode, to determine the quantity of each alloy phase in the plating layer on the basis of the quantity of electricity consumed in each range of the potentials during the electrolysis.
A second aspect of the present invention provides a method for determining a quantity of a xcex6 phase in a plating layer of an alloyed hot-dip galvanized steel sheet, wherein the method includes performing constant potential electrolysis within the range of a potential of xe2x88x92940 to xe2x88x92920 mV vs. SCE in aqueous zinc sulfate-sodium chloride using an alloyed hot-dip galvanized steel sheet as the anode to determine the quantity of a xcex6 phase in a plating layer on the basis of the quantity of consumed electricity.
A third aspect of the present invention provides a method for determining each quantity of a xcex6 phase and a xcex41 phase in a plating layer of an alloyed hot-dip galvanized steel sheet, wherein the method includes performing constant potential electrolysis within the range of a potential of xe2x88x92940 to xe2x88x92920 mV vs. SCE in aqueous zinc sulfate-sodium chloride using an alloyed hot-dip galvanized steel sheet as the anode to obtain the quantity of a xcex6 phase in a plating layer on the basis of the quantity of consumed electricity, and then subjecting the alloyed hot-dip galvanized steel sheet, which is the anode, to constant potential electrolysis within the range of a potential of xe2x88x92900 to xe2x88x92840 mV to obtain the quantity of a xcex41 phase in the plating layer on the basis of the quantity of consumed electricity.
A fourth aspect of the present invention provides a method for determining each quantity of a xcex6 phase, a xcex41 phase, and a xcex93 phase in a plating layer of an alloyed hot-dip galvanized steel sheet, wherein the method includes performing constant potential electrolysis within the range of a potential of xe2x88x92940 to xe2x88x92920 mV vs. SCE in aqueous zinc sulfate-sodium chloride using an alloyed hot-dip galvanized steel sheet as the anode to obtain the quantity of a xcex6 phase in a plating layer on the basis of the quantity of consumed electricity, subjecting the alloyed hot-dip galvanized steel sheet, which is the anode, to constant potential electrolysis within the range of a potential of xe2x88x92900 to xe2x88x92840 mV to obtain the quantity of a xcex41 phase in the plating layer on the basis of the quantity of consumed electricity, then subjecting the alloyed hot-dip galvanized steel sheet, which is the anode, to constant potential electrolysis within the range of a potential of xe2x88x92830 to xe2x88x92800 mV to obtain the quantity of a xcex93 phase in the plating layer on the basis of the quantity of consumed electricity.
A fifth aspect of the present invention provides a method for evaluating the sliding property of an alloyed hot-dip galvanized steel sheet, wherein the method includes performing constant potential electrolysis within the range of a potential of xe2x88x92940 to xe2x88x92920 mV vs. SCE in aqueous zinc sulfate-sodium chloride using an alloyed hot-dip galvanized steel sheet as the anode to determine the alloyed hot-dip galvanized steel sheet has a satisfactory sliding property if the quantity of consumed electricity is small.
In the fifth aspect, the quantity of consumed electricity is preferably 0.5 C/cm2 or less, and the electrolysis is preferably determined to be terminated when the current density reaches 5 xcexcm/cm2.
The term xe2x80x9cvs. SCExe2x80x9d used herein as the unit of potential represents a potential based on the saturated calomel electrode.